With the advent of high-speed data transmission systems, various technologies have emerged to provide increased reliability and robustness of the data transmitted and received. One technique that is commonly used to accomplish this is trellis coded modulation (“TCM”), where such data is trellis encoded and then mapped or modulated onto any of various, standardized signaling formats or constellations for transmission, such as pulse amplitude modulation (“PAM”) or quadrature amplitude modulation (“QAM”).
To simultaneously achieve both coding gain and decision feedback equalization performance, such data for transmission also may be “precoded” (known as Tomlinson-Harashima (“Tomlinson”) precoding), to provide a pre-equalization functionality prior to data transmission. For such precoding, a reciprocal training mode is utilized between two data communication devices (“DCDs”) which are in communication with each other through a communication channel. In this training mode, an equalizer within the receiver of each DCD determines a plurality of linear filter coefficients to, among other things, correct for various channel impairments, such as varying frequency responses and various types of noise. Each given DCD then transmits its own, individually determined equalization coefficients, referred to in the art as “Tomlinson” or “Tomlinson-Harashima” coefficients, to the other DCD, such that the other DCD then may utilize these coefficients in its transmission of data to the given DCD.
Different equalization and precoding schemes have been utilized or are being proposed for precoding in various high-speed data transmission systems, and are being included as recommendations or standards, such as the T1.418 standard accredited by the American National Standards Institute (“ANSI”). For example, for communication systems such as high bit rate digital subscriber line (“HDSL”) systems, no precoding is utilized, while for the next or second generation HDSL as proposed in the ANSI T1.418 standard, generally referred to as “HDSL2”, Tomlinson-Harashima precoding is utilized. Tomlinson-Harashima precoding schemes are also utilized for systems such as G.shdsl of the International Telecommunications Union (“ITU”) recommendation G.991.2 and some other forms of digital subscriber line systems generally referred to as “xDSL”.
Particular difficulties arise in high speed data transmission in environments such as xDSL, due to, among other things, the use of cables comprised of many sets of twisted pair wires, in which a single twisted pair (two wires) is used for full duplex transmission over a given communication channel. In some environments, the noise power spectral density may be substantially “white”, with noise power distributed rather flatly across the frequency spectrum. For environments such as xDSL, however, with as many as fifty twisted pairs of lines providing service, cross-talk impairments become very significant, with significant correlated noise, and having a noise power distributed very unevenly across the frequency spectrum. For example, for xDSL environments, data reception includes not only reception of a transmitted signal, but also reception of as many as 49 crosstalk noise contributions and other noise couplings between the various pairs of wires. In addition, there is a significant variability in the types of anticipated crosstalk. This potential correlated noise poses various unique problems for accurate data transmission and reception.
Another difficulty which arises for noise reduction with xDSL is its use of trellis decoding. Depending upon the depth of the trace or path back through the trellis, such trellis decoding may introduce a significant delay, typically on the order of several (or more) symbol time periods, which cannot be tolerated with prior art noise cancellers or noise predictors. For example, in Gadot et al. U.S. Pat. No. 5,513,216 and Wang U.S. Pat. No. 5,604,769, both entitled “Hybrid Equalizer Arrangement for Use in Data Communications Equipment”, a noise predictor is illustrated for use with an equalizer, during a training mode prior to data transmission, to determine coefficients for transmission-side precoding. Neither the Gadot patent nor the Wang patent, however, illustrate or otherwise disclose how such a noise predictor should be used with a trellis decoder for ongoing adaptation during actual data transmission, particularly in light of the potential delays involved in trellis decoding.
As a consequence, a need remains for an apparatus, method and system for correlated noise reduction or whitening, utilizing a noise predictor which may be implemented as part of or with an equalizer, to substantially reduce correlated or non-white noise in data reception and transmission. Such a noise predictor should be readily adaptive, converging quickly to optimal linear filter values without excessive training time. In addition to providing precoding coefficients, such an apparatus, method and system for correlated noise reduction should also provide adaptive functionality during data transmission, to adjust to potentially changing noise levels and spectral distributions. Such an apparatus, method and system for correlated noise reduction should provide noise whitening during data transmission within a system utilizing a trellis decoder, both with or without transmission-side precoding. Lastly, such a noise predictor should be capable of implementation as a linear adaptive filter.